This invention relates to the combustion of coal and in particular to the reduction of mercury (Hg) and nitrogen oxides (NOx) in flue gas generated during coal combustion.
Emissions from coal combustion can contain oxides of nitrogen (NOx) and volatile metals such as mercury (Hg). There is a long felt need to reduce Hg and NOx in gaseous emissions from coil-fired boilers and other industrial coal combustion systems.
As mercury volatizes during coal combustion, it enters the flue gas generated by combustion. Some of the volatized mercury can be captured by coal fly ash and removed via a particulate collection system. The volatized mercury that is not captured in the particulate collection system, or by some other control system, passes into the atmosphere with the stack gases from the coil boiler. It is desirable to capture as much of mercury in flue gas before the stack discharge.
Mercury volatizes as elemental mercury (Hg0) during coal combustion. Oxidized mercury (Hg+2) is more easily collected by emission control devices than is elemental mercury. Oxidization of mercury is a known technique to capture mercury and remove it from flue gases. As flue gases cool, mercury is partially oxidized by chlorine which is present in coal and released during combustion. It is believed that most oxidized mercury (Hg+2) in flue gas is present as mercury chloride (HgCl2). Oxidation of mercury occurs in combustion gas-phase reactions and on the surface of fly ash. It is believed that mercury oxidation on the surface of fly ash is a predominant channel of mercury oxidation.
Oxidized mercury (HgCl2 or Hg+2) is water soluble and is easily adsorbed on high carbon fly ash or activated carbon. The oxidized mercury captured by fly ash may be collected with the ash and removed via a particulate collection system. Oxidized mercury is also easily removed by wet scrubbers that are used to control sulfur dioxide (SO2) emissions. Mercury control is generally most effective when the mercury in flue gas is mostly oxidized.
Controlling mercury emissions is complicated because mercury is present in flue gases in several different forms, such as elemental mercury (Hg0) and oxidized mercury (Hg+2) . Mercury changes forms throughout the combustion process. To effectively control mercury emissions, a control system should take into account the form or speciation of the mercury present in the flue gases at the location of the control system in the flue gas stream. In addition, it would be beneficial if mercury emission controls reduced NOx in stack gases discharged by a power plant.
There is a special need to control mercury emission from boilers firing low-rank coals, such as coal from the Powder River Basin (hereinafter PRB) and lignite coals. These low-rank coals represent a significant portion of the coal energy market, especially in the U.S. These low-rank coals often have low sulfur content, which minimize SO2 emissions. Mercury emissions from the burning of low-rank coals tends not to oxidize because of the low chlorine (Cl) content of the coal and the presence of other constituents in the coal that tend to suppress Hg oxidation. Chlorine assists in the oxidation of mercury in flue gas. The low chlorine levels in low rank coal results in relatively high levels of elemental mercury in the flue gas from low rank coal. There is a long felt need to reduce the levels of elemental mercury in flue gas in coal fired plants, especially when low-rank coals are fired.